Manufacturers of alkaline batteries have invested substantial amounts of time and money in the effort to develop mercury-free batteries. The first batteries to incorporate mercury-free constructions were the AA, C and D standard alkaline batteries which are tubularly shaped such that the height of the battery greatly exceeds its diameter. These batteries have historically used the largest quantities of mercury per cell and are the most common sizes and types purchased by consumers. Therefore, removing mercury from these batteries was the best way to significantly reduce the quantity of mercury that is currently entering the general waste stream when used alkaline cells are thrown away.
In addition to developing mercury-free standard alkaline batteries, battery manufacturers have also sought to develop miniature battery constructions that are free of mercury. Miniature batteries are generally disc shaped such that the diameter of the batter is greater than the height of the battery. The anodes usually contain zinc powder and the cathodes may contain oxides of manganese, silver oxide, mercuric oxide, etc. While a layman may believe that the technology used to produce mercury-free standard alkaline batteries can be used to produce mercury-free miniature alkaline batteries, the manufacturers of miniature alkaline batteries have found that additional technology had to be developed before mercury-free miniature alkaline cells could be manufactured on a commercial basis.
Constructional differences between miniature alkaline batteries and standard alkaline batteries have forced battery manufacturers to develop processes and techniques that pertain to miniature cells. One of the key differences between standard alkaline batteries and miniature alkaline batteries is the shape of the collector. Standard alkaline batteries generally use an elongated or pin-shaped collector. The vast majority of the collector's surface is contained within and therefore exposed to the cell's electrolyte. One end of the collector usually protrudes slightly beyond the cell's interior in order to contact a separate part which serves as one of the battery's terminals. In contrast, miniature batteries typically use a generally disc shaped part that serves as both a collector and a terminal. This part, commonly referred to as an anode cup, must have one surface that can withstand exposure to the cell's caustic electrolyte and a second surface that can withstand exposure to the cell's external environment. Manufacturers of miniature alkaline cells have traditionally used multilayered anode cups in order to provide a part with two different surfaces. A substrate, such as steel, was generally coated on one side with nickel and on the opposite side was coated with a layer of copper. These multilayered anode cups have worked well for miniature alkaline cells that contain mercury in the anode mix. However, testing has shown that mercury free miniature alkaline cells that used the conventional multilayered anode cups generated excessive amounts of gas which lead to leakage of electrolyte. Therefore, there is a need for an anode cup that can be used in mercury-free miniature alkaline batteries.
Japanese unexamined application 01-307161 is directed to a mercury-free alkaline cell wherein the negative electrode's collector is coated with indium and/or lead which can be supplied by any method such as electroplating.
U.S. Pat. No. 4,500,614 discloses an alkaline cell having an anode prepared by amalgamating an alloy powder made of zinc and at least two metals selected from the group consisting of gallium, indium and thallium. The metals are incorporated in the cell to reduce the amount of mercury required to prevent corrosion of the zinc in an alkaline electrolyte which causes generation of hydrogen gas and subsequent leakage of the electrolyte.
German Patent 1,086,309 discloses an alkaline zinc cell in which an indium compound is added to the electrolyte and/or indium metal is alloyed with refined zinc so as to protect the zinc against corrosion in an acidic, neutral or alkaline electrolyte.
Japanese Publication No. 1958-3204, published Apr. 26, 1958, recites that the addition of 0.0001% to 2.0% indium can be added to pure zinc base alloy containing one, two, or more of the metallic elements Fe, Cd, Cr, Pb, Hg, Bi, Sb, Al, Ag, Mg, Si, Ni, Mn, etc., to form a zinc alloy which has a high corrosion resistance and which is suitable for use in primary cells.
It is an object of the present invention to provide a method for producing a coated cup-shaped terminal for housing a zinc-containing electrode of a miniature alkaline cell.
It is another object of the present invention to provide a miniature alkaline cell with a zinc-containing electrode that is substantially free or completely free of mercury and wherein the surface of the cup-shaped terminal contacting the zinc-containing electrode has an electrodeposited coating comprising an element, such as indium, and an overlayer which is free of indium.
These and other objects of the invention will be apparent from the following description.